1. Technical Field
The present invention relates to a time certifying server, a reference time distributing server, a time certifying method, a reference time distributing method, a time certifying program, and communication protocol program, each relating to, for example, issuance of a time stamp for an electronic document.
2. Background Art
With the rapid development of information technology in recent years, so-called “paperless office policy” has been promoted, under which official or private documents are prepared each in the form of an electric document, obtained by digitizing its original document.
Also, in order to improve legal status given to such electronic documents, the electronic document law is about to be enforced.
In such electronic documents as prepared and stored in the above-mentioned way, what is important is time certification (date certification), such as certification of an issue date, for which a time stamp system for performing time certification is used.
FIG. 13 is a block diagram explaining a system configuration of a conventional time stamp system.
A time stamp system 100 includes a time distributing server 101 provided in a time distributing office, a time stamp server 102 provided in a time certification office, and client terminals 103, 103, etc.
The time stamp server 102 is a server which issues a time stamp, and each of the client terminals 103 is a terminal unit which receives a time stamp thus issued. A procedure of issuing a time stamp is as follows:
The client terminal 103 transmits a hash value of an electronic document, which is a target of time certification, to the time stamp server 102.
The time stamp server 102 receives a hash value from the client terminal 103, generates an electronic signature with respect to a time output from an internal clock installed therein and the hash value thus received, and transmits this as a time stamp to the client terminal unit 103.
The time distributing server 101 is a server which manages and distributes a time, which serves as a reference for a time used in the time stamp system 100. The time distributing server 101 has, for example, an atomic clock for measuring a reference time, and distributes a reference time, output from the atomic clock, to the time stamp server 102.
The time stamp server 102 receives a reference time from the time distributing server 101, and adjusts the internal clock by using this reference time.
In order to prevent doctoring of the reference time and time delay in a communication path, encryption is applied to the communication path provided between the time stamp server 102 and the time distribution server 101 by using a technique such as Secure Socket Layer (SSL), and Network Time Protocol (NTP) is used as a communication protocol. Note that the NTP is a protocol which is standardized for time distribution, and can correct time delay caused in the communication path.
Note that FIG. 13 shows only one time stamp server 102, but a configuration may be adopted in which a plurality of time stamp servers 102 are connected to the time distributing server 101.
As a technique of configuring such a time stamp system, there is the following system and method of providing a reliable third party clock and a reliable local clock.    Patent Document 1: Published Japanese Translation of PCT application 2003-519417
In this technique, a master clock system is provided as the time distributing server 101, and a local clock system is provided as the time stamp server 102, wherein the master clock system checks the local clock system, or performs other operations.
With this configuration, the master clock system (time distributing server 101) distributes a reference time to the local clock system (time stamp server 102), and further audits whether or not a time is doctored in the local clock system.